1. Field of the Invention
The present invention broadly relates to hot rolling methods and facilities for metallic sheets, and more particularly, to a method and apparatus for producing a hot rolled steel sheet.
2. Description of the Related Art
In a process of hot rolling a sheet, such as a process for hot rolling a steel sheet, the rolled sheet should be rolled at a temperature as low as possible but above a required level. In general, when the temperature is higher, more energy is lost per unit time period, and the temperature more rapidly decreases. Accordingly, in view of efficient utilization of thermal energy, hot rolling should preferably be carried out at a temperature as low as possible but still capable of securing product quality.
As described above, a decrease in the temperature of the rolled sheet during the rolling process can be a cause of some problems. In a rolling process including major steps of roughing rolling and finish rolling, the rolled sheet should be at a temperature which is higher than a predetermined level after the finish rolling step. Further, since deformation resistance should be restricted so as not to exceed limitations concerning performance of a finish rolling mill, the temperature should be controlled so as not to be lower than a predetermined level before the sheet is sent to the finish rolling mill. Hitherto, due to such requirements, an initial temperature was determined taking into account any decrease in the temperature during the roughing rolling step.
In methods disclosed in Japanese Unexamined Patent Publications No. 59-92114 and No, 62-214804, edge portions of sheets, which particularly readily grow cold, are re-heated by transverse-type induction heating.
Front and rear end portions of sheets also readily grow cold, and Japanese Unexamined Patent Publications No. 1-321009 and No. 4-3371, disclose methods in which the front and rear end portions of a sheet are heated over an entire width by an edge-heating apparatus used in the methods disclosed in Japanese Unexamined Patent Publications No. 59-92114 and No. 62-214804, while moving the apparatus in a sheet-width direction when the front and rear end portions pass over it.
As a method in which heating is performed over the entire sheet-width, Japanese Unexamined Patent Publication No. 51-122649 discloses a method in which a transverse-type induction heater is arranged, to preheat a steel sheet for a subsequent process, as close as possible to an apparatus for the subsequent process.
However, although the initial temperature can be lowered to some extent, a drastic resolution has not been achieved yet by the above-described methods, disclosed in Japanese Unexamined Patent Publications No. 59-92114, No. 62-214804, No. 1-321009, and No. 4-33715, which compensate for the temperature decrease heavily occurring in the edge portions of sheets or at the front and rear ends thereof in order that the initial temperature before rolling should be set low.
In such circumstances, a method has been considered in which a heater is intermediately arranged, the initial temperature is aggressively lowered to reduce a thermal energy loss in an early stage of rolling, and rolling is carried out while being accompanied by re-heating performed at an appropriate position.
Induction heating can be considered as an easily-practiced technique for intermediate heating. However, transverse-type induction heating which is described in Japanese Unexamined Patent Publication No. 51-122649 has some problems such as a complicated apparatus due to a necessity of providing a means for controlling a coil gap, and excessive heating of edge portions of sheets.
As described in Japanese Unexamined Patent Publication No. 51-122649, the re-heating apparatus is usually arranged as close as possible to the apparatus of the subsequent step. According to such an arrangement, however, since a surface temperature of a sheet is high, thermal energy added by induction heating is readily lost in a case where the subsequent step, such as descaling or rolling, can be a cause of cooling from the sheet surfaces.
In a process for producing a hot rolled steel sheet, since the sheet: is heated and rolled in a high temperature range from 800 to 1300.degree. C., oxide scales are generated on surfaces of the sheet. If such scales are left on the surfaces, the scales are pressed during rolling so that they are included in the surface portion of the sheet, and the resulting hot rolled steel sheet will have scale flaws.
As is generally known, scale flaws are classified into two types described below.
(1) Inclusion Scales
Inclusion scales are generated as follows: PA1 Scales which have not been completely removed in a descaling process preceding a finish rolling mill are pressed into the surface portion of the sheet during a finish rolling process. PA1 Particulate scales are generated as follows: PA1 Secondary scales which have been generated after the descaling process preceding the finish rolling mill are pressed into the surface portion of the sheet during the finish rolling process. PA1 (a) a roughing rolling mill to roughing-roll a heated slab into a sheet bar; PA1 (b) at least one solenoid-type induction heater to re-heat the sheet bar; and PA1 (c) a finish rolling mill to finish-roll the re-heated sheet bar. PA1 (a) a roughing rolling mill to roughing-roll a slab having a predetermined temperature into a sheet bar; PA1 (b) at least one solenoid-type induction heater to re-heat the sheet bar over an entire width of the sheet bar; PA1 (c) a descaling apparatus for descaling oxide scales on surfaces of the sheet bar; and PA1 (d) a finish rolling mill to finish-roll the sheet bar, PA1 wherein the solenoid-type induction heater and the descaling apparatus are arranged between the roughing rolling mill and the finish rolling mill in an order of the roughing rolling mill, the solenoid-type induction heater, the descaling apparatus, and the finish rolling mill.
(2) Particulate Scales
In order to prevent the generation of inclusion scales, a surface temperature of a sheet before descaling should be set at a high value. The higher the surface temperature of the sheet is, the greater the amount of generated scales becomes and the larger the internal stress of the scales becomes, since the temperature difference between before and after the descaling process becomes large, and a thermal stress generated on interfaces between the scales and the sheet also becomes large.
Japanese Unexamined Patent Publication No. 6-269840 discloses a method in which surfaces of a sheet are heated using gas burners at a position just preceding a descaling apparatus.
On the other hand, in order to prevent the generation of particulate scales, the surface temperature of the sheet after descaling should be restricted to inhibit the generation of secondary scales.
The surface temperature of the sheet before descaling should preferably be as high as possible to prevent the generation of inclusion scales, while it should preferably be as low as possible to prevent the generation of particulate scales. Accordingly, there is an optimum temperature range in which neither type of scales are generated, and the temperature of the sheet before descaling should be controlled so that it falls within the optimum range in which neither type of scales are generated.
In a method disclosed in Japanese Unexamined Patent Publication No. 6-269840, temperature control is substantially impossible since surfaces of a sheet are heated using gas burners. Since a temperature of the sheet may be too low in some cases while it may be too high in other cases, it is difficult to prevent the generation of both types of scales.
Further, the use of gas burners is also accompanied by problems such as those described below.
(1) The productivity is lowered because a time period for preparation such as preheating is required for ignition and extinction of the gas burners.
(2) The working environment readily deteriorates due to generation of combustion gases.